The present invention relates to a ceramic circuit board, more particularly to a ceramic circuit board which enables the reduction of an influence of stress caused by heat cycles during the operation of the circuit board, to increase the durability and reliability of the ceramic circuit board.
In recent years, various types of ceramic circuit boards have been widely used as a circuit board for a hi-power transistor module or a high-speed, high-power switching power source modules. The ceramic circuit board comprises a ceramic base board composed of ceramic sintered body and a metal plate such as a copper plate integrally bonded onto a surface of the ceramic base board, the metal plate having high-thermal conductivity and high-electrical conductivity.
Further, as a material for the ceramic base board, aluminum nitride, silicon nitride, silicon carbonate or the like are now widely used because of their excellent electrical insulation and thermal conductivity.
The ceramic circuit board having a circuit composed of the conductive metal plate such as copper plate is generally manufactured in accordance with a DBC method (Direct Bonding Copper method) wherein the metal plates are directly bonded onto the ceramic base board, or an active metal brazing method wherein the metal plates are bonded onto the surface of the ceramic base board through a brazing material containing an active metal, such as 4A family elements and/or 5A family elements (hereinafter referred to as "active metal") listed in the periodic table. Namely, the active metal is selected from the group consisting of Ti, Zr, Hf, V, Nb and Ta. The brazing material containing such an active metal activates the surface of the ceramic base board. In other words, the brazing material imparts wettability to the bonding surface of the ceramic base board, to thereby realize a high-bonding strength.
As a concrete method of forming the circuit, the following methods are known: i.e., the methods may include one wherein a metal plate having a predetermined circuit pattern previously formed by press working or etching treatment are bonded onto the ceramic base board; or a method wherein the circuit patterns are formed after a raw material of the metal plate is bonded onto the ceramic base board.
FIG. 27 shows a construction of the ceramic circuit board manufactured in accordance with the DBC method described above. The ceramic circuit board is manufactured by the following processes. That is, a raw copper material is punched out to form a plurality of copper circuit plates 2A, 2B and 2C each having a predetermined shape (circuit pattern). The copper circuit plates 2A, 2B and 2C are, then, arranged and contacted to both surface sides of the ceramic base board 1 composed of alumina (Al.sub.2 O.sub.3) or an aluminum nitride (AlN) sintered body or the like having a thickness of about 0.3 to 3 mm, to thereby form a board assembly. Then, the board assembly is heated up to a predetermined temperature to generate an eutectic liquid phase to thereby impart wettability to the surface of the ceramic base board 1. Subsequently, the board assembly is cooled and the eutectic liquid phase is solidified whereby the copper circuit plates 2A, 2B and 2C are directly bonded onto the ceramic base board 1.
Further, among the plurality of the copper circuit plates 2A, 2B and 2C, an edge portion of the copper circuit plate 2A is provided with a terminal connecting port 3 to which a terminal of a LSI circuit element or module to be loaded onto the ceramic base board 1 is connected.
As shown in FIG. 28, the terminal connecting port 3 is previously formed by punching out the copper circuit plate 2A by means of a press-punching machine and subsequently, bending the edge portion so as to have a U shaped section. After the copper circuit plate 2A is integrally bonded onto the surface of the ceramic base board 1 by employing the DBC method, a free edge side of the terminal connecting port 3 is retained so as to be raised from the surface of the ceramic base board 1. A terminal 4 of a module (not shown) is connected onto an upper surface of the terminal connecting port 3 through a solder 5.
In order to improve the solder wettability with respect to terminal connecting port 3 composed of copper, and to increase a bonding strength between the terminal 4 and the terminal connecting port 3, a nickel metallizing layer 6 having a thickness of about 2 to 6 .mu.m is formed on each surface of the copper circuit plates 2, 2A to 2C.
As a result, even in a case where the terminal 4 is moved and deformed vertically due to a thermal expansion or the like as indicated by an arrow in FIG. 28, the deformation or replacement of the terminal connecting port 3 is absorbed by the bending of the terminal connecting port 3 around a base portion thereof. Accordingly, the stress caused by the thermal expansion is effectively prevented from being concentrated to the ceramic base board 1.
On the other hand, FIG. 29 shows a construction of another conventional ceramic circuit board manufactured by utilizing the active metal brazing method.
The ceramic circuit board shown in FIG. 29 comprises a ceramic base board 11 and a copper circuit plate 12 integrally bonded onto one surface of the ceramic base board 11 through a bonding layer 13 mainly composed of silver (Ag), copper (Cu) and an active metal such as titanium (Ti) or the like.
A backing copper plate 14 for preventing a thermal deformation of the ceramic base board 11 is bonded onto the other surface side of the ceramic base board 11 through a similar bonding layer 13 containing the active metal.
Further, in a module as a product, a semiconductor chip 15 such as a Si chip is bonded onto the surface of the copper circuit plate 12 through a solder layer 16, and required wires are provided so as to electrically connect the semiconductor chip 15 to the copper circuit plate 12. Also a heat sink 17 is bonded onto the back surface side of the backing copper plate 14 through a solder layer 16.
The active metal brazing method is utilized as a method of bonding a ceramic base board and a metal circuit plate to each other, the method comprising the steps of: preparing a paste by mixing a binder such as an organic component or the like and a solvent into active metal powder such as Ti or the like; pattern printing the paste onto a surface of the ceramic base board 11; arranging the copper circuit plate onto a surface of the ceramic base board 11 in along with the printed pattern to form an assembled body; and heating the assembled body in an inert gas atmosphere (i.e., Ar gas or N.sub.2 gas atmosphere or the like) or vacuum to thereby thermally bond the copper circuit plate 12 onto the ceramic base board 11.
The backing copper plate 14 is also bonded onto the ceramic base board by utilizing the same method as described above.
According to the method described above, Ti reacts with nitrogen (N) or oxygen (O) to generate a TiN or TiO, at the same time, Cu and Ag react with the copper component to generate a eutectic bonding material whereby the copper circuit plate 12 and the backing copper plate 14 are firmly bonded onto the ceramic base board 11 respectively.
Any one of the ceramic circuit boards that are manufactured in accordance with the DBC method or the active metal brazing method has a simple structure, so that there can be provided advantages of enabling the miniaturization of the circuit board and the realization of high-density loading, furthermore, to simplify the mounting process, or the like.
In particular, the active metal brazing method is directly applicable to various ceramic materials, and simultaneously, has an advantage of imparting an excellent reproducibility of fine circuit pattern in comparison with that of DBC method. Therefore, the active metal brazing method will become widely used in some particular fields.
However, the conventional ceramic circuit boards shown in FIGS. 27 and 28, have a nickel plating layer formed on the copper circuit plates so as to enhance the solder wettability, so that a problem may be created in that the copper circuit plates are hardened and likely to cause breakages. In fact, the copper material is originally soft and rich in elasticity at the stage prior to being provided with the nickel plating layer. However, after forming of the nickel plating layer, the copper circuit plate will become hard due to the high-hardness of the nickel material per se. Therefore, in particular, the bent portions P.sub.1 and P.sub.2 of the terminal connecting port 3 become brittle, so that repeated bending strength of the bent portions P.sub.1 and P.sub.2 will be disadvantageously decreased.
In the conventional case described above, a radius of curvature R.sub.1 and R.sub.2 of the bent portions P.sub.1 and P.sub.2 is small, about 0.1 mm, so that bending stress is concentrated to the bent portions P.sub.1 and P.sub.2 to easily cause cracks which result in the terminal connecting port 3 being liable to be broken in a short period of time. This reduces the reliability of the ceramic circuit board.
More particular, in the ceramic circuit board having such a terminal connecting port, temperature variation caused by the ON/OFF switching operation of the power source is repeatedly applied onto the terminal connecting port. Accordingly, a problem may be created in that the crack or breakage is more liable to occur at the terminal connecting port.
On the other hand, in the case of the conventional ceramic circuit board manufactured by employing the active metal brazing method, the bonding layer 13 for bonding the ceramic base board 11 and the copper circuit plates or the like (i.e., the copper circuit plate 12 and the backing copper plate 14), or the bonding layer 14 for bonding the copper plates 12, 14 and the semiconductor chip or the like (i.e., the semiconductor chip 15 and the heat sink 17) do not always impart or realize a sufficient bonded state.
Namely, in the process where the ceramic base board 11 and the copper circuit plates 12 and 14 or the like are thermally bonded to each other, if a dewaxing operation is effected uncompletely, carbon (C) contained in the bonding material is not completely removed whereby residual carbon remains within the bonding layer 13.
The residual carbon C is liable to react with the active metal such as Ti, resulting in disadvantageously forming TiC which interrupts an essential reaction to be required between the Ti component and N or O contained in the ceramic base board 11. Therefore, various problems are created in that: non-bonded portions are formed in the bonding layer 13; or even in a case where the bonding is effected completely, the bonding strength thereof may be lowered. In some adverse cases, a proportion of the non-bonded area with respect to the an entire bonding surface will reach almost up to 30%.
In a module assembling process, when a high-temperature solder is placed onto the surfaces of the copper circuit plates 12, 14 and subsequently, the semiconductor chip 15 and the heat sink 17 are arranged onto the surface of the copper circuit plates 12 and 14, gases such as air and atmospheric gas are likely to be involved between the semiconductor chip 15 and the copper circuit plate 12, or between the heat sink 17 and the backing copper plate 14 whereby there may be posed a case where the gases remain in the solder layer 16.
When the gases remain in the solder layer 16, various problems may occur in that: a pore 18, so-called, a solder void may be formed as schematically shown in FIG. 29, to thereby reduce the bonding strength. Further, thermal resistance of the final product will increase to thereby deteriorate heat releasing capability. Additionally, the semiconductor chip 15 may easily be broken, thus reducing the reliability of the module or the like.
Furthermore, when the ceramic circuit board having the semiconductor chip 15 thereon, which is manufactured by employing the active metal brazing method, is assembled into a power-transistor module or the like, a terminal of the module is soldered to a part of the circuit of the copper circuit plate or the like so as to secure an electric continuity between the circuit and an electric power source.
In such a module described above, a temperature difference is necessarily caused in accordance with ON-OFF switching operations of the power source, and when the temperature difference is repeatedly applied onto the ceramic circuit board for a long period of time, as shown in FIG. 29, cracks 19 are liable to occur at a side of the ceramic base board 11. Particularly, in a case where the aluminum nitride base board is used as a ceramic base board, the mechanical strength of the aluminum nitride is lower by nature than that of other ceramic materials, so that the cracks 19 are more likely to occur.
Further, the terminal is moved and deformed in accordance with the ON-OFF switching operations of the power source, so that mechanical stresses are also applied to the ceramic circuit board. When the thermal stresses due to the temperature difference and the mechanical stresses are applied onto the ceramic circuit board for a much longer period of time, there may be posed various problems such that cleavage fractures disadvantageously occur from the portion of the cracks 19. Thus resulting in peeling-off the copper circuit plate 12 from the ceramic base board 11, and the thermal resistance of the module per se is increased, thus inducing malfunctions of the module or the like.
In order to solve the aforementioned problems inherent in the prior art, an object of the present invention is to provide a ceramic circuit board which enables the increase of the durability of the ceramic base board, and secures a sufficient bonding reliability of the metal circuit plate.